The portable and cordless trend is in a rapid progress recently in various electronic appliances such as audio-video systems and personal computers, and as the driving power sources for them, enclosed type batteries of high energy density and excellent load characteristic are demanded. Conventionally, as the driving power sources of small and thin portable electronic appliances, square cells using square battery case cans with a flat rectangular cross section were mainly used because it was easier to install into the power supply unit of the appliances. In this square cell, a plate group laminating positive and negative pole plates is put in a square battery case can, and the opening is sealed by laser.
Lately, on the other hand, a wide attention is drawn to the lithium battery which is high in voltage and energy density, and long in storage life, among other features. It is, however, difficult to fabricate such lithium battery of flat shape by using the same square battery case can, as the driving power source for small and thin portable electronic appliances. That is, in the lithium battery, the plate group is formed by spirally winding positive and negative plates. Therefore, when putting this plate group into the battery case can of square cross section, the plate group is likely to be deformed due to mismatching of the sectional shapes, and there is also a possibility of current leak when charging and discharging are repeated.
Hence is proposed an elliptical battery in a tubular form with a bottom, using an elliptical battery case can of which cross sectional shape in the orthogonal direction to the tubular center is a flat elliptical shape. This elliptical cell, as compared with the square cell, is easier to process the battery case can, and is capable of preventing deformation of the shape of the spiral plate group contained in the battery case can. Therefore, it has many features, including high reliability and large cell capacity per volume, and it is particularly expected to increase in demand rapidly as the driving power source for small and thin portable electronic appliances.
In the battery, however, after inserting a sealing plate into an opening in the battery case can through an insulating gasket, generally, the opening edge of the battery case can is folded inward to seal. Accordingly, on the inner circumference near the opening of the battery case can, an annular support portion for mounting and supporting the sealing plate is formed, and this annular support portion is formed by the portion bulging to the opposite side of an annular groove by indenting the annular groove along the opening on the outer circumference near the opening in the battery case can. As for this annular support portion, favorable processing methods suited to mass production have been already established in the cylindrical cells and square cells, but effective processing method suited to mass production has not been proposed yet in the elliptical cells.
For example, in the cylindrical cells, the annular support portion is formed by the processing methods shown in FIG. 5 and FIG. 6. In FIG. 5 and FIG. 6, (a) is a partially cut-away sectional view, and (b) shows sectional views along line A--A and line B--B in (a), respectively. As shown in FIG. 5, the bottom portion of a metallic battery case can 1 of a cylindrical form with a bottom is inserted into a case holding portion 2, a protruding portion 3a of a case support portion 3 is fitted into the opening of the battery case can 1 to press a step portion 3b against the opening side, and the battery case can 1 is pressed from both sides by the case holding portion 2 and case support portion 3 to be fixed, and are also supported rotatably about the tubular center by the case holding portion 2 and case support portion 3 supported rotatably.
The cylindrical battery case can 1 is rotated at a constant speed by force as torque is transmitted from a rotation drive source to the case holding potion 2. In this state, as shown in FIG. 5, a groove forming roller 4 being rotatably supported and opposite at a specific position near the opening on the outer circumference of the battery case can 1 is moved toward the battery case can 1 as shown in FIG. 6. Being pressed to the outer circumference of the groove forming roller 4 at a constant pressure, an annular groove 7 is indented in the outer circumference of the battery case can 1, and an annular support portion 8 is formed by the bulging portion on the opposite side of the annular groove 7 in the inner circumference of the battery case can 1.
In the square cell, as proposed in Japanese Laid-open Patent No. 6-44951, for example, the portion near the opening of the metallic battery case can of square tubular form with a bottom is expanded, and the sealing plate is mounted and supported in the annular support portion composed of a stepped portion formed in this manner.
When forming an annular support portion in the elliptical battery case can, a processing method shown in FIG. 7 may be considered. That is, a elliptical battery case can 10 is rotated in the direction of arrow in FIG. 7 (a) while pressing and holding from both sides in the tubular center direction, by the case holding portion and case support portion, same as shown in FIG. 5 and FIG. 6, and a groove forming roller 11 is pressed against to a specified position on the outer circumference of this elliptical battery case can 10. In FIG. 7, the elliptical battery case can 10 located at the reference position (a) is rotated by 30.degree., 40.degree., 60.degree., and 90.degree., respectively, in (b) to (e). The groove forming roller 11 is always pressed at a constant pressure against the elliptical battery case 10 by a pressurizing device (not shown) such as air cylinder, and a constant pressurizing force F acting on the fulcrum of rotation of a roller shaft 12 of the groove forming roller 11 is shown in a vector diagram in a direction linking the center of the groove forming roller 11 and center of the elliptical battery case can 1, in a length coinciding with the radius of the groove forming roller 11, for the sake of simplicity of drawing.
Therefore, the groove forming roller 11 is, as clear from FIG. 7 (a) to (e), pressed while rolling against the outer circumference of the elliptical battery case can 10 rotated always at a specified position, and when rotated by 90.degree. by tracing the shape of the outer circumference, it is advanced by distance D toward the battery case can 10. Thereafter, every time the battery case can 10 is rotated by 90.degree., the groove forming roller 11 is moved reciprocally to move in and out of the battery case 10 in a range of distance D. Thus, the groove forming roller 11 is moved reciprocally in a range of a relatively long distance D to the battery case can 11, and moreover, with respect to the battery case can 10 rotating at a constant speed, the rotational angle of the battery case can 10, that is, the moving speed changes considerably much depending on the contact position of the outer circumference of the battery case can 10. Further, as compared with the pressurizing force "F" acting on the rotation fulcrum of the roller shaft 12 of the groove forming roller 11, the component "W" of the force acting in the orthogonal direction of the pressurizing force "F" due to reaction by the battery case can 10 changes significantly depending on the rotational angle of the battery case can 10, and the direction is inverted every time the rotational angle exceeds 90.degree..
Accordingly, when the battery case can 10 is rotated at high speed for the purpose of enhancing the productivity, the tracing performance of the groove forming roller 11 to the battery case can 10 is extremely impaired, and the groove forming roller 11 is momentarily spaced from the battery case can 10, and the both members repeat collision, which causes vibration. As a result, the annular groove or the annular support potion cannot be processed at high precision. Therefore, the processing method shown in FIG. 7 cannot be applied in mass production of elliptical cells. If the annular support portion for mounting the sealing plate could be processed in the elliptical battery case can 10 at high precision and high productivity, the elliptical cells having various such excellent features would be mass produced.
It is hence an object of the invention to present a processing apparatus capable of forming the annular support portion for mounting a sealing plate on an elliptical battery case can at high precision and high productivity, and a structure of an elliptical battery using such elliptical battery case can.